This invention relates to a synchro-to-digital converter for converting a rotational angle of a rotor of a synchro to absolute digital values.
Heretofore, an A-D converter, pulse generator, or the like has been used in a servo-system wherein a rotational angle of a synchro is converted into digital values.
More specifically, the A-D converter is coupled through gear trains and the like to a rotating shaft for obtaining digital values corresponding to the rotational angle of the rotating shaft, and the digital values thus obtained are thereafter transmitted from the detecting position to a control device provided at a remote station. However, such an arrangement has required a number of lead wires extending between the detecting position and the control device in a remote station, and this requirement has constituted a drawback of the conventional device especially when the distance between the detecting position and the control device is considerably long.
Furthermore, the A-D converter employed in the servo-system is inevitably of high precision, and the required ambient conditions therefor are strict. For this reason it is not desirable to use an A-D converter in a detecting position where the ambient conditions are inferior.
On the other hand, a pulse generator generates pulses in accordance with the rotation of the shaft, and these pulses are counted by a counter or the like provided in the control device. Since this method inevitably provides an incremental mode of operation, the detection of an absolute position is not possible in principle, constituting a drawback when a pulse generator is utilized. Furthermore, the detecting element used with a pulse generator as well as with and A-D converter is ordinarily formed by a device having brushes, a photoelectric element of a noncontact type, or a magnetic device, whereby the working conditions thereof are restricted, and the device having brushes is accompanied by the problem of durability.
A synchro-system which is resistant to severe ambient conditions has also been used for detecting the rotational angle of a shaft. When this synchro-system is used for detecting the rotational angle, the required number of transmitting wires can be reduced to three, and an absolute detection of an angle is also possible when a synchro-to-digital converter is provided at the remote station to be operated in conjunction with the synchro-system.
Various types of synchro-to-digital converters have been proposed (for instance, in U.S. Pat. No. 3,071,324). A simple example of a synchro-to-digital converter has included a Scott-T transformer at its input stage, wherein the input rotational angle .theta. is converted into signals representing sin .theta. and cos .theta., and the entire 360.degree. -range of the rotational angle .theta. is divided into 4 or 8 sectors depending on the signs or magnitudes of these signals, thus obtaining higher output bits representing the rotational angle .theta.. The sine and cosine signals in each of the sectors are then selected with respect to the modes of the output bits and supplied to a subsequent switch resistor network. According to the polarity of the output of the switch resistor network relative to a reference signal, a series of clock pulses are generated.
The clock pulses are then counted, and the thus counted results are utilized for controlling the switch resistor network which constitutes a D-A converter. That is, an output voltage is obtained from the switch resistor network depending on the counted results (in the form of digital values) obtained from the counter, whereby the rotational angle .theta. of the input shaft can be determined by reading out the counted results (in a digital form) at the time the output voltage from the switch resistor network becomes zero.
However, the above described example of the conventional synchro-to-digital converter including a Scott-T transformer has a drawback in that, because the operational range of sine and cosine functions obtained therefrom is divided into sectors such as 4 = 2.sup.2 or 8 = 2.sup.3, the digital outputs from the synchro-to-digital converter are limited to the pure binary code, and outputs in the form of binary coded decimal code or ordinary decimal outputs cannot be obtained.
Furthermore, in some specific conditions of the polarity of the sine and cosine functions or the magnitude of the voltage, the operational range of the sine and cosine functions cannot be divided into more than 8 sectors, that is, less than .pi./4sectors, whereby the conversion error becomes too large if linear digital conversions are simply carried out in these sectors. For this reason, a nonlinear digital conversion simulating the sine curve or cosine curve must be effected in these sectors, thus necessitating a D-A converter of an arrangement using a precision resistance network, whereby the production cost is increased.
When it is desired to further divide each sector, a counter is utilized, and the counter is driven from the lower bits. Such an operation requires a considerably long operation period, and the conversion speed of the synchro-to-digital converter is thereby restricted.
In addition, both the positive and negative sides or halves of a reference wave are used for the polarity discrimination, which fact further restricts the controlling speed of the counter.
Although the representation of an absolute value of the rotational angle .theta. is possible in the conventional synchro-to-digital converter, the absolute position is determined beforehand by the wiring between the synchro transmitter and the synchro-to-digital converter, thus making it impossible to zero-adjust the thus obtained digital values. For this reason, another differential synchro is inserted between the synchro transmitter and the synchro-to-digital converter for the realization of the zero-adjust, this constituting a further drawback of the conventional synchro-to-digital converter.